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The filter has been changed, but after the prob started, it is weird indeed the filter is a cannister type and the only posible leaks could be at the pipe in and out connections,so, tommorow I shall start replacing fuel pipe from filter onwards, and i will use clear pipe so i can keep an eye on things, This is the reason i asked if there was a non-return valve in the bulb as if there was , this could faulty, if there isn't ,then I can see no way that the bulb would go hard, If my reasoning makes any sense to you.